Hi @NitPicky
Welcome to the forum. Scvaler has a number of functions that may help you with this:
On the MAIN page if you click on the TRIADS" button in Section B you will get a drop down menu, which will enable you to add 7ths, 9ths, 11ths or 13th chords.
Then click on the SUGGEST button
to open a dialogue giving suggested chords for the next block.
A second approach would be to use the CHORD page to edit and add chords.
The tutorial Searching for Chord Variations and Extensions may help you with these approaches.